Except for laptop systems, personal computers are almost always configurable by means of changing and adding adapter cards which are plugged into the system bus. The system bus includes a number of lines including various signals, together with electrical ground lines and lines supplying power at one or more voltage levels. An important part of the development of computer systems has been the development of standardized, efficient versions of the system bus. The specifications of a bus, which are collectively called an "architecture," describe the type of connectors to be used, the types of signals, including timing conditions, the voltage levels, and the connector positions used to provide voltage levels and grounds, and the allowable physical conditions of cards. The most popular type of system bus is now known as the ISA (Industry Standard Architecture) bus. Another significant type of bus is the local bus architecture of the PCI (Peripheral Component Interconnect) bus.
FIG. 1 is a block diagram of a prior-art personal computer including both a PCI local bus 10 and an ISA system bus 12. The processor 14 is connected to cache memory 16 and to a bridge/memory controller 18 by means of a processor bus 20. The bridge/memory controller 18 in turn provides an interface to both the PCI bus 10 and the random access memory 22. The PCI bus 10 is connected to the ISA system bus 12 through an interface circuit 24. The PCI bus 10 also includes a number of card slot connections 26, through which connections with adapter cards configured particularly for use with a PCI bus are made. Similarly, the ISA bus 12 includes a number of card slot connections 28, through which connections with adapter cards configured particularly for the ISA bus are made.
FIG. 2 is a fragmentary vertical cross-section of a computing system having the configuration of FIG. 1. The system board 29 includes ISA connectors 30, 32, through which the ISA bus 12 is provided, and connectors 34, 36, through which the PCI bus 10 is provided. Each connector 30, 32, 34, 36 includes a number of contact springs 37 and a key 37a. A number of ISA adapter cards 38 are plugged into the ISA connectors 30,32, while a number of PCI adapter cards 40 are plugged into the PCI connectors 34, 36. The PCI connectors 34, 36 are displaced inward, in the direction of arrow 39, from the ISA connectors 30, 32, and the PCI connectors 34, 36 are smaller, having closer spacings between adjacent electrical contacts.
The system board 29 is mounted within a frame 42 including a slotted bracket 44 through which connections are made between external cables 46 and the adapter cards 38, 40. Each I/O connector 48 on an ISA adapter card 38 for attachment to an external cable 46 having a cable connector 49 extends in the direction of arrow 50 from the card 38, with electrical connections usually being made between various conductive layers in the card and pins extending through holes within the card material. Each ISA adapter card 38 also includes a card bracket 54, extending along the internal side 56 of the slotted bracket 44 to close the slot 58 within the bracket 44 when the card 38 is installed within an associated ISA connector 30, 32. The connectors 48 extend through holes within the card brackets 54. Some ISA adapter cards do not need connections to external cables 46, and thus include brackets without holes for cable connectors. A central slot 60 within the slotted bracket 44 is covered in this way by either a card bracket 54 of an ISA card, plugged into ISA connector 32, or alternately a card bracket 62 of a PCI card 40 plugged into PCI connector 36. To make this possible, the card bracket 62 and connector 66 of each PCI card 40 extends opposite the direction of arrow 50 from the PCI card 40.
Each card bracket 54 is fastened to an associated ISA card 38 by means of a pair of rivets or screws 70 extending through tabs 72 formed as parts of the bracket 54. Similarly, each card bracket 62 is fastened to an associated PCI card 40 by means of a pair of rivets or screws 74 extending through tabs 76 formed as parts of the bracket 62. Slots 58, 60 which are not occupied by brackets 54 of ISA cards 38, or by brackets 62 of PCI cards 40, are closed by filler plates 80. Each bracket 54, 62 and each filler plate 80 is held in place by a screw 82 engaging a threaded hole (not shown) within a bracket locating surface 84 of the slotted bracket 44.
The ISA bus was introduced in an 8-bit version running at 4.77 MHz with the original IBM Personal Computer in 1981. In 1984, a 16-bit version of the ISA bus was introduced with the IBM Personal Computer/AT, running at 8 MHz. Multiplying this speed by the 16-bit data width, indicates that the bus runs at 128 megabits per second. However, since the bus requires at least 2 cycles to transfer data, the actual data transfer rate is at most 64 Megabits per second, or 8 MBytes per second.
The specification for the PCI bus was released in June, 1992, and updated in April, 1993. Information is transferred across the PCI bus at 33 MHz and at the full data width of the processor. For example, when the PCI bus is used with a 32-bit processor, the bandwidth is 1,056 Megabits per second, or 132 MBytes per second. The PCI bus, implemented as shown in FIG. 1, is called a local bus, because it provides a way for external devices, such as adapter cards, to access the processor bus, which is local to the processor. The PCI bus used in this way is also called a mezzanine bus, because it adds another layer to the traditional configuration, in which the system bus was connected to the processor bus through bus controller chips.
The primary advantage of the ISA bus is its widespread availability, being included as the system bus within almost every "IBM-compatable" personal computer. Almost any adapter card function which is available for personal computers is available with an ISA interface. The primary disadvantage of the ISA bus is its lack of speed. This problem can be traced to the original Personal Computer, in which the system bus extending to I/O card slots operated at the same speed as the processor bus. Since then, with advances in processor chips, processor bus speeds have significantly increased, but it has been impossible to provide significant increases in the speed of the ISA bus because of the large inventory of adapter cards which cannot operate at significantly higher speeds. Nevertheless, the ISA bus continues to be used, even in high-performance personal computer systems, because of its relatively low cost, because it can be used to take advantage of the large number of available adapter cards, and because many I/O functions occur at speeds slower than that of the ISA bus, so that an increase in bus speed would not result in a similar increase in system performance.
An important advantage of the PCI bus is its speed. In the example described above, with the PCI bus used with a 32-bit processor, its data transfer rate is 16.7 times that of the ISA bus. Furthermore, the PCI bus has been used as the model for the Intel "Plug and Play" specification, which has eliminated the jumpers and switches used to set the configuration of ISA cards, with PCI cards being instead configured through the use of software.
Given an adequate supply of adapter cards of each type, the personal computer of FIGS. 1 and 2 can operate either with only ISA adapter cards 38 attached to the ISA bus 12 or with only PCI adapter cards 64 attached to the PCI bus 10. There is a particular advantage in connecting cards and peripheral devices (through cards) which require high data rates, such as audio, motion video, and graphics functions, to the PCI bus 12, while standard, lower-speed I/O functions are driven through the ISA bus 12. At any rate, present trends point toward increased usage of the PCI bus and toward decreased usage of the ISA bus. Eventually, the ISA bus may be eliminated. If this occurs, anyone buying an adapter card engaging the ISA bus may expect not to be able to use it when he eventually purchases a computing system having only the PCI bus. Therefore, what is needed is an adapter card which can be easily used with the ISA bus, and alternately with the PCI bus.
A number of examples of adapter cards which can alternately be plugged into two different bus architectures are found in the U.S. patent art. For example, U.S. Pat. No. 4,885,482 describes a first embodiment of such a card having edge connector tabs extending along opposite longitudinal card edges near opposite ends of the card. Differing types of card brackets are installable on the opposite ends of the card, with the appropriate bracket for a type of card (i.e. ISA or Micro Channel) being installed on the card end adjacent to the associated card edge connector. Thus, this embodiment of an adapter card is rotated 180 degrees about a central axis of rotation extending perpendicularly from the plane of a surface of the card. This embodiment does not include a provision for an end connector, and is thus suitable only for cards not requiring such connectors. A second embodiment of the card includes a single edge connector tab, for example of the ISA type, extending along a longitudinal edge, and a male-pin connector extending outward from an end of the card. This type of card is suitable for use in systems providing either the ISA bus or, alternately, the VMEbus. In a third embodiment, an edge connector tab extends from a single longitudinally-extending edge of the card near each end of the card, so that the card can be removed from one bus and rotated 180 degrees about a vertical axis of rotation before it is plugged into another bus. If a connector for an external cable is required, similar connectors must be placed at both ends of the card. The card bracket is mounted solely by means of screws extending into the bracket.
U.S. Pat. No. 5,163,833 describes a circuit card having edge connector tabs extending along opposite longitudinal card edges near the same end of the card, so that the circuit card can be flipped over to plug into either of two types of bus. A removable, repositionable connector board assembly is provided at this end of the card for mounting communications connectors to external cables in a position accessible from the back panel of the personal computer. The connector board assembly includes both the external connector(s) for cable connection(s) and an internal connector for electrical connection with the remaining portion of the circuit card. In the example shown, when the connector board assembly is repositioned, the bracket is changed between the type of bracket used with the ISA bus and the bracket used with the Micro Channel bus.
U.S. Pat. Nos. 4,885,482 and 5,163,833 further discuss electrical considerations, including a method for automatically determining which bus is plugged in and active, and operation in emulation modes.
What is needed is a circuit card which can be plugged into either of two types of busses with external cable connections being usable in the same way regardless of the type of bus to which the card is connected. It is further desirable to eliminate a requirement for additional connections between the circuit card and a separate connector board which is repositionable on the end of the circuit card after removal and rotation. Also, what is needed is a circuit card which can be used either in a conventional ISA slot, with an external cable connector being displaced in a first direction from the card, or in a conventional PCI slot, with an external cable connector being displaced opposite this first direction from the card.